The focus of the proposed research is to elucidate the molecular-genetic mechanisms that are responsible for specifying the mesodermal progenitor cells that will give rise to heart. It has been shown that the functions of the homeobox gene, tinman, and the secreted factors wingless (wg) and hedgehog (hh) are required for the formation of the heart progenitor cells, indicating a critical role in mesodermal patterning. The aim is to understand how tinman and the wg/hh signals determine the formation and developmental fate of the cardiac mesoderm. The specific aims are: 1) To define the requirements for the tinman gene in heart formation: a) to understand the precise temporal and spatial requirements of tinman by using specific promoters to drive tinman expression in transgenic flies and by generating mosaic flies of wild-type and mutant tissue; to test whether vertebrate and fly heart development is similar by expressing tinman-related genes from various vertebrate species in tinman mutant flies and examining their ability to restore heart development. 2) To understand the regulation of tinman expression in the heart by expressing wg/hh in the mutant background of the other and by generating genetic mosaics. The tinman promoter will be dissected in vivo to identify heart-specific elements and to isolate new transcriptional regulators that bind to heart- specific enhancers of the tinman gene.